This invention relates to an apparatus for the heat treatment of the necks of thermoplastic preforms and, more particularly, to an apparatus for increasing the mechanical and heat resistance of the neck of a polyethyleneterephthalate (PET) preform.
In the packaging field, bioriented PET containers are currently widely used for the commercial distribution of carbonated beverages (lemonades, sodas, carbonated fruit juices, etc.), non-carbonated beverages (non-carbonated fruit juices), juices and/or sauces. These containers must be able, without undue deformation, to be subjected to relatively severe conditions of heat such as those encountered during the filling of the container with a hot liquid, or occurring during the pasteurization of the liquid contents of the container.
This wide use of PET containers is due to the fact that bioriented PET has good mechanical and heat resistance, a good appearance, and high chemical inertia to the products contained in the containers, and it forms an effective barrier against the gases contained in the liquids and against the oxygen in the air (conservation of the products contained without oxidation).
The containers are obtained from a preform, which is generally an amorphous PET, and is brought to a sufficient temperature and then introduced into a mold and pressed against the walls of the mold through the introduction of a gas under pressure. During this blow-molding, the parts of the container which are stretched are bioriented. However, the parts of the container which are not bioriented, in particular the neck and possibly the base, do not have the same heat and mechanical resistance as the remainder of the container. This is a problem because the neck must retain the same size characteristics to allow sealed closing once the containers are filled with a hot liquid or undergo a pasteurization treatment.
It is known to improve the heat and mechanical resistance of the neck of a PET container by subjecting the neck to a heat treatment to increase its degree of crystallization by increasing the density of its spherolitic structure. However, since the neck of the container is provided with a thread intended for screwing on a cap, the thickness of the neck is not uniform and, during the heat treatment, local deformations occur which hinder or prevent sealed closing.
It is also known to introduce, during the heat treatment of the neck of the container, a cylindrical mandrel having a diameter smaller than that of the inside diameter of the neck, and then shrinking the neck by cooling it with blown air to bring the interior of the neck into contact with the mandrel. While this avoids the above-noted disadvantages, providing the calories necessary for the heat treatment cannot be strictly localized on the neck and local deformations can appear on the bioriented body of the container.
It is further known to apply a reinforcing heat treatment, not to the neck of the manufactured container but to the neck of the preform, prior to its blow-molding. For this purpose, a heating block made of a material which is a good heat conductor, and having a cavity whose shape corresponds to that of a preform neck, is applied to the neck of a preform for a sufficient length of time to cause complete crystallization of the neck. The heating lock is traversed by a sliding cylindrical mandrel which is inserted into the neck of the preform. However, in this device, there is poor contact between the heating block and the neck of the preform because the neck has a complex shape due to its screw thread. In addition, the top of the thread which is in contact with the cavity of the heating block is deformed because it receives a high heat flow. Finally, with this device, it is necessary to have one type of heating block for each type of neck.